1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device with a trench-type device isolation insulating film.
2. Description of the Related Art
In a semiconductor device including a nonvolatile memory such as a flash memory, in which a large number of element (memory cells) are arranged on a semiconductor substrate, trench-type device isolation insulating films for isolating the elements are formed thereon. These trench-type device isolation insulating films are formed as STI films, and trenches are formed to extend into the semiconductor substrate to a predetermined depth, and the trenches are filled with the insulating films.
FIG. 1 is a plan view showing a conventional flash memory, and FIG. 2 is a circuit diagram thereof. The trench-type device isolation insulating films STI are formed on the semiconductor substrate in an island manner in a predetermined interval in the X-direction to extend in the Y-direction, as shown in FIG. 1. Memory cells M are formed on the semiconductor substrate between the trench-type device isolation insulating films STI in the X direction, and a plurality of island-shaped floating gates FG of the memory cells M are arranged in the Y-direction, and a source region S and a drain region D of each memory cell M is formed to put the floating gate FG between them. The source regions S are formed as source lines SL to extend in the X-direction, and bit line contacts BC are formed on the drain regions D to connect them to bit lines BL, as shown in FIG. 2. Also, capacitive insulating films are formed on the floating gates FG, and control gates CG as word lines WL are formed on the capacitive insulating films to extend in the X-direction, although they are not shown in the Figure.
FIGS. 3A to 3C are cross sectional views showing the conventional flash memory. Referring to FIGS. 3A to 3C, a method of manufacturing the conventional flash memory with the trench-type device isolation insulating film STI. As shown in FIG. 3A, a tunnel oxide film 102, a floating gate film 103, a buffering oxide film 104 and a nitride film 105 as a stopper film are successively laminated on the surface of a semiconductor substrate 101. Subsequently, these films are selectively etched to form trenches 106 having a predetermined depth and extending into the semiconductor substrate 101. Next, as shown in FIG. 3B, an insulating film 111 is formed to fill the trenches 106 and to cover the nitride film 105. Then, as shown in FIG. 3C, the insulating film 111 is polished by a CMP (Chemical Machine Polishing) method by utilizing the nitride film 105 as a stopper film, so as to flatten the surface of the insulating film 111. Then, the nitride film 105 and the oxide film 104 are etched, and the insulating film 111 is etched so that the insulating film 111 is remained only in the trenches 106 to form a trench-type device isolation insulating film STI. Thereafter, a capacitive insulating film 107 and a control gate film 108 are formed and these films are patterned to a predetermined pattern. Subsequently, impurity is implanted in the semiconductor substrate 101. Thus, the memory cells of the conventional flash memory are formed.
Japanese Laid Open Patent Application (JP-P2002-110780A) discloses a conventional manufacturing method of a trench-type device isolation insulating film. In this conventional example, a first isolation film is formed in trenches which are formed in a semiconductor substrate, and the trenches are partially filled through an annealing process. Subsequently, the trenches are completely filled with a second isolation film. Then, a CMP method is carried out for flattening the surface to form the trench-type device isolation insulating film. In this conventional example, the first isolation film is partially buried to relax a stress due to difference in thermal expansion coefficient between the semiconductor substrate and the insulating film, to prevent defects in the trench-type device isolation insulating film, and to improve the device reliability.
In the above method of manufacturing a trench-type device isolation insulating film, when an insufficient buried portion with the insulating film is produced, a defective semiconductor device is manufactured. For example, in the trench-type device isolation insulating film STI formed by the manufacturing method shown in FIGS. 3A to 3C, there would be a case that a part of the trenches 106 is clogged by a minute foreign substance Z generated upon the forming of the insulating film 111, so that the trench 106 located right under the foreign substance Z is not filled with the insulating film 111, as shown in FIG. 4A. The foreign substance Z is produced through separation of a part of an insulating film from the inner wall of a film forming apparatus (not shown), floats inside the film forming apparatus, and adheres on the surface of the semiconductor substrate.
The insulating film 111 is formed from an oxide film (hereinafter, to be referred to as an HDP (High Density Plasma) film) formed by a HDP method in many cases. Since the HDP method has a high anisotropy in the growth of the insulating film, it is difficult to fill a trench portion shaded by the foreign substance with the insulating film, failing to conduct the burying step on such a trench portion.
When such a non-buried state with the insulating film occurs in one portion of the trench, a capacitive insulating film 107 and a control gate film 108 are formed in the trench 106 in the subsequent steps, as shown in FIG. 4B. As a result, the control gate film 108 for the control gate CG faces the semiconductor substrate 101 only through the thin insulating film 111 and the capacitive insulating film 107. For this reason, when a predetermined voltage is applied to the control gate CG in the data write or read operation into or from the memory cell, the capacitive insulating film 107 is broken down because of electric field applied between the control gate CG and the semiconductor substrate 101, resulting in leakage between them. This does not cause any problem in a peripheral transistor that operates in a normal power supply voltage. However, this would cause an insufficient write operation, an insufficient read operation and an insufficient erase operation in the flash memory that uses a high voltage, and needs detection of a minute current difference. Thus, it is difficult to provide a flash memory with high reliability.
This problem would be caused in the above conventional example. For example, if the foreign substance is adhered in a manner to cover one portion of the trench upon the forming of the first insulating film, the first insulating film is not formed, and the second insulating film is also not formed thereon. As a result, the trench portion is not filled with any insulating films.
In recent years, elements have been developed to have a finer structure, and the trench-type device isolation film being miniaturized. For this reason, fine dusts and foreign substances causes serious problems, although having not caused any problems conventionally.
Also, when a trench-type device isolation insulating film STI is formed by using trenches with a small size, and insulating films, there is a problem that both sides of the trench opening are made in contact with each other during the forming step of the insulating film to be buried in the trench, to close the opening of the trench. In this case, the same state as shown in FIG. 4A is caused.